1. Field of the Invention
The present invention generally relates to a coating system including a clear coat layer and a sealant. More specifically, the present invention relates to a coating system that is modified to enable threshold adhesion strength to be achieved between glass and a substrate bonded through the coating system without affecting appearance of the clear coat layer even in the event of below-specification film builds of the clear coat layer in the coating system.
2. Description of the Related Art
Bonding of stationary glass, such as a windshield, to a vehicle body during a vehicle assembly process presents many quality and safety issues to automotive manufacturers. For example, motor vehicle safety standards (MVSS), such as MVSS 212, require a threshold adhesion strength between glass and vehicle bodies for ensuring that the glass remains bonded to the vehicle body, especially in the event of a crash or rollover situation, to minimize the incidence of injury to passengers of the vehicle resulting from detachment of the glass from the vehicle body. In fact, in crash and roll-over situations, the glass may provide additional support to a roof of the vehicle and may thereby help to prevent injuries resulting from compaction of the roof of the vehicle. Therefore, the glass preferably remains bonded to the vehicle body.
During the vehicle assembly process, a coating system is formed on the vehicle body prior to the point in the process at which the glass is bonded to the vehicle body. The coating system typically includes a clear coat layer, a base coat layer, and other sub-clear coat layers that are known in the art. The glass is bonded to the coating system of the vehicle body via a sealant that is typically applied in the form of a bead to the clear coat layer. Thus, the adhesion strength between the glass and the vehicle body is dependent upon the interaction between the sealant and the clear coat layer within the coating system.
Many factors may contribute to the robustness of the bond between the glass and the vehicle body, including the formulation of the sealant, the formulation of the clear coat layer, base coat layer, and other sub-clear coat layer(s) within the coating system on the vehicle body and inter-layer adhesion between those layers, compatibility of the formulations used for the sealant, the clear coat layer, the base coat layer, and the sub-clear coat layer(s) within the coating system, film build of the clear coat layer, the base coat layer, and other sub-clear coat layer(s) within the coating system, and the ability of the clear coat layer, base coat layer, and other sub-clear coat layer(s) in the coating system to cure at lower than normal times and temperatures.
While numerous methods of achieving the threshold adhesion strength have been used in the past, satisfaction of the adhesion strength thresholds has generally been accompanied by detrimental effects to the aesthetic qualities (e.g. appearance) and/or physical properties of the coating system or has required manual application of a reactive primer or a tape masking between the clear coat layer and the sealant.
Coating compositions that are used to form the clear coat layers are generally formulated based on a desire to achieve certain aesthetic qualities and/or physical properties in the resulting coating system, such as acceptable appearance, durability, resistance to scratching and degradation due to UV light, environmental etching, etc. For example, crosslinkable carbamate-functional polymers have been found to provide particularly desirable properties to the coating system in terms of resistance to environmental etching. Such crosslinkable carbamate-functional polymers are known in the art.
One strategy that has been used in the past to maximize the adhesion strength between the sealant and the coating system has been to modify the resin used to form the clear coat layer or to add additional additives into the resin to provide functional groups for reacting with the sealant. The resin is sensitive to modification or addition of additives, which usually negatively impacts one or more of the aesthetic qualities of the resulting clear coat layer. The effect of the additives or modifications on the resin is unpredictable and varies depending upon the type of resin. Further, when additives are used, automotive manufacturers require sufficiently high amounts of the additives to be added to the resin to enable threshold adhesion strength to be achieved even for below-specification film builds of the clear coat layer. The high amounts of the additive in the resin typically magnify the detrimental effects to the aesthetic qualities of the resulting clear coat layer. Automotive manufacturers also require that the resin used to form the clear coat layer be capable of “low temperature cure”, or curing at lower than normal curing temperatures, to account for process variance without sacrificing aesthetic quality or safety performance of the coating system. Modification or the resin or addition of additives may affect the ability of the resin to satisfy the low temperature cure requirements, thereby possibly resulting in a higher incidence of below-specification coating systems.
With regard to the manual application of the reactive primer between the sealant and the clear coat layer, known reactive primers have reactive groups that react with the sealant and the clear coat layer, and much care must be taken to avoid application of the reactive primer onto the clear coat layer outside of specific regions on the vehicle body where the glass is to be bonded. Automated application of the reactive primer is generally not feasible, and laborers are typically required to apply the reactive primer, thus adding cost and time to the vehicle assembly process.
In view of the foregoing, there is a desire within the automotive and coatings industries to provide solutions to the problems associated with bonding stationary glass to coating systems, especially coating systems having a clear coat layer formed from carbamate-functional resins. More specifically, there is a desire to achieve threshold adhesion strength, in accordance with MVSS standards, between the glass and the vehicle bodies while accounting for below-specification film builds of the clear coat layer and low-temperature curing of the coating system, all while minimizing detrimental effects to the aesthetic qualities of the coating system.